Among image processing apparatuses designed to input image data by scanning and reading original images, an apparatus having the ACS (Auto Color Select) function of automatically determining whether a read original image is a monochrome or color image is known.
In the above conventional image processing apparatus, however, in executing ACS operation, a prescan for ACS operation must be performed before the execution of a real scan to read the image data. The processing time in this case is longer than that in the case where only a real scan is performed without ACS operation, resulting in a decrease in productivity of the apparatus as a whole.